Silicon Pixel Readout for a TPC

1. Silicon Pixel Readout for a TPC ALCPG 2007 - Fermilab 23 October 2007 Jan Timmermans NIKHEF

2. Micro Patterned Gaseous Detectors GEM • High field created by Gas Gain Grids • Most popular: GEM & Micromegas Micromegas Use ‘naked’ CMOS pixel readout chip as anode

3. Timepix pixel • Timepix chip: • 256x256 pixels • pixel: 55x55 μm2 • active surface: 14x14 mm2 CERN Timepix chip (1st version) produced Sept. 2006 Available for use in detectors since Nov. 2006

4. Timepix in gaseous detectors • With Micromegas grid or GEM stacks • Wafer postprocessing: • Integrated grid (Ingrid) • Enlarged pixels • Discharge protection: high-resistive (~1011) Ω·cm amorphous Si layer (20 μm thick) on top of CMOS chip (later maybe also high-resistive grid)

5. New Ingrid developments and results • Process improvement: grids much flatter • Extremely good energy resolution: 13.6 % FWHM with 55Fe in P10 • Removal of Kβ 6.5 keV line: 11.7 % @ 5.9 keV in P10 • New wafer masks: hole pitches down to 20 μm with various diameters and gaps • Investigate Micromegas geometry • Test of the ion backflow theory • Until now: 1 μm thin Al but can now be increased to 5 μm by electrolysis Expect less damaged from sparks Kα escape Kβ escape 13.6 % FWHM Gap: 50 μm; Hole picth: 32 μm,Ø: 14 μm Kβ-filtered spectrum with Cr foil 11.7% FWHM

6. InGrid ion backflow measurements • Phenomenon depends on: • Avalanche charge distribution • Funnel size • therefore on the gas and grid geometry • Q density in the funnel decreases with the avalanche transverse diffusion • Funnel size decreases with the field ratio and hole pitch • Backflow fraction reaches a (minimum) plateau • Occurs when ions backflow through neighboring holes • Simulation predicts this to occur at σ/p = 0.5

7. Full post-processing of a TimePix · Timepix chip + Micromegas mesh: Moiré effects + pillars · Timepix chip + SiProt + Ingrid: MESA+ “Uniform” IMT Neuchatel “counting” mode

8. NIKHEF setup (> 22 Aug. 2007) Next-3 Next-1,2 “old”

9. Status of Timepix usage at NIKHEF TPX operated 1 month in He iC4H10 1st fully post processed MPX 13 dec. B05 with 3 µm SiProt & Micromegas in He 20% iC4H10 24 jan. Switch to Ar 20% iC4H10, chip died after 2 days 20 mar. MediPix2 with 3 µm SiProt & InGrid operated 4 days in He 17 apr. C08 with 3 µm SiProt & Micromegas & guard electrode (G.E.) in He 25 jul. Stop C08 after 3 months of continuous operation in He E09 with 20 µm SiProt & InGrid placed in NEXT-1 chamber in He 22 aug. A06 with 20 µm SiProt & Micromegas placed in NEXT-2 chamber in He 04 sep. Flush NEXT-2 (A06) with Ar, stable operation for >40 days! 23 sep. Flush NEXT-1 (E09) with Ar, same nice results 26 sep. Introduce Thorium in NEXT-2 (A06), provoke discharges Recording alpha’s tracks & even more… TPX operated 3 months in He iC4H10 1st fully post processed TPX TPX operated in Ar iC4H10 ALL STILL WORKING !!

10. Irradiation: 90Sr source 90Sr He/Iso (80:20) Timepix + 20 μm Siprot + Ingrid in Next-1 Time mode 118 μs shutter courtesy David Attié

11. time The “typical” track Timepix + 20 μm thick Siprot + Ingrid A “long” cosmic track Stable operation in He iC4H10 Will 20 µm SiProt be enough to operate in Ar? (picture is 14x14 mm2)

12. Stable operation in Argon too! Time mode After 2 weeks of cosmic event recording, it was time for a definitive assessment whether 20 µm SiProt is enough to protect against discharges…

13. Final assessment: spark-proofness • Provoke discharges by introducing small amount of Thorium in the Ar gas • Thorium decays to Radon 222 which emits 2 alphas of 6.3 & 6.8 MeV • Depose on average 2.5.105 & 2.7.105 e- in Ar/iC4H10 80/20 at -420 V on the grid, likely to trigger discharges Charge mode During ~3 days, some 5.104 alpha events recorded in 1% of which …

14. … discharges are observed ! For the 1st time: image of discharges are being recorded Round-shaped pattern of some 100 overflow pixels Perturbations in the concerned column pixels • Threshold? • Power? Chip keeps working !!

15. Few slides from Saclay and Freiburg(/Bonn)

16. TimePix/Micromegas chamber: first light at Saclay (D. Attié, P. Colas, E. Delagnes, M. Riallot, A. Giganon) • Small TPC chamber with a 6 cm height field cage • Timepix chip • + SiProt 20 μm • + Micromegas • Ar/Iso (95:5) • Time mode 55Fe, z = 25 mm

17. TimePix Chip: 14 mm Freiburg (+Bonn) DESY Test Beam June 2007 (before the end of HERA…) Several mixtures studied: Ar/CO2 (70:30) He/CO2 (70:30) He/CO2/C4H10 (68:30:2) Ar/He/CO2 (60:10:30) TDR Two different GEM types tested: Standard 100x100mm2 GEMs with 140µm hole pitch New 24x28mm2 GEMs with 50µm hole pitch TIMEPIX: 14* 14 mm2 “Mixed Mode” (MM) operation records Time-Over-Threshold (TOT) and TIME e- beam DESY II AGAIN VERY ROBUST (TIMEPIX) OPERATION FOR BOTH GEM TYPES GEM: 10 * 10 cm2 Trigger (scint.) & Si-telescope

18. Tracks recorded with small pitched GEMs Ar/CO2 Gain with small pitched GEMs at DVGEM 346V comparable to DVGEM 403V with standard GEMs.

19. Dt - transverse diffusion coefficient • n - number of primary electrons per cluster • y - drift length • σ0 15-25µm el cl Spatial resolution Resolution studies Clusteringmethod Time resolution Time resolution was evaluated in MM-operation. A correlation between TOT-maximum and Time was used to correct for time-walk problems (typically 2-3 counts). Proves robustness of cluster separation algorithms. For s0 good agreement between experiment and simulations. Dt2/nelcl is in fair agreement.

20. x New Technical Developments New GEM type (Bellazzini) Post processing on a single chip before after z Guard ring (also on lower side) Active area 28x24mm2 • Nominal outer hole diameter 30µm in copper • Inner hole sizes are as small as 17µm-21µm in the Kapton • Pitch of holes 50µm • Projected in x  43µm • Projected in z  25µm 55x55µm2 110x110µm2 • larger pixels available • Idea: collect more charge per larger pixel  reduction of effective thresholdexpected • FMF in Freiburg is going to prepare a TimePix after first tests with MediPix2

21. Summary • A lot of progress made in last ‘year’; not mentioned many details on track resolution studies and on signal development • Part of the technology is ready: • Very good energy resolution for Ingrid devices • Ion backflow at the few per-mil level at high field ratio • Discharge protection seems working for Ingrid devices • Robust operation with GEM devices (without protection) Next: • Build larger multi-chip detector systems with fast readout

22. Backup slides

23. A “scratch” occurred during production Ingrid; Loose parts removed. Ingrid working!

24. 200 Charge [pC] 700 Measurement of discharge “spectrum”(signal from Ingrid recorded on digital scope)

25. Further Developments RELAXD project (Dutch/Belgian) NIKHEF,Panalytical,IMEC,Canberra: • Chip tiling: large(r) detector surfaces (2x2, 2x4 chips) • Through Si connectivity: avoiding bonding wires • Fast readout technology (~5 Gb/s) Somewhat slower progress than expected: Still hope for “through Si vias” (with Medipix chips) later this year!

26. Substructure due to GEM hole pitch standard GEM =>120μm Is the resolution of a cluster yet affected by the finite pitch of the holes? Test runs are taken recently with different orientation with respect to the track and with smaller pitched GEMs (80μm). Results are expected to be available soon.

27. GEM structure (new) after rotation by 90° the observed spatial signal from the GEM pitch vanishes no peak FFT spatial signal